Well, there is SO much information available about the Habsburgs! You can do an internet search. Just enter "Habsburg" into the Google search engine, and you'll find all kinds of things.
Something involving your grandfather or great-grandfather would have occurred during the time of Emperor (in German: Kaiser) Franz Joseph I (1830-1916; reigned 1848-1916). So you might also find something useful if you do a search for "Franz Joseph I" as well.
The following two webpages might also be of interest to you (although not related to your mother's line):http://pages.prodigy.net/ptheroff/gotha/harrach.htmlhttp://harrach.nwy.at/
(available in German and in English)
One possibility is that the person you're interested in did not as such change his surname, but rather, married a woman by the name of Greatrex (a relatively common name in the Midlands) and decided that they would use her family name rather than his. Have you looked for marriage records yet?